Anticoagulant therapy with heparin, ethyl biscoumacetate, and bishydroxycoumarin was used in 317 patients with manifestations of cerebral vascular disease. A group of 94 patients were classified as having intermittent insufficiency in the vertebral-basilar system, and in 90 of these the attacks stopped completely soon after the anticoagulant effect of the drugs became demonstrable by laboratory tests. A group of 107 patients with irreversible vertebral-basilar thrombosis had a mortality rate of only 8% on anticoagulant therapy as compared with a rate of 58% reported in 31 similar patients who did not receive anticoagulants. A group of 85 patients with intermittent insufficiency in the carotid system were treated with anticoagulants, and in 82 the characteristic attacks were stopped. A group of 31 patients with actively advancing carotid thrombosis went on to hemiplegia in only 6 % of the cases, as compared with 35 % of a reported series of 17 similar patients who did not receive anticoagulants. Anticoagulant treatment is preventive rather than reconstructive, but does alter favorably the natural history of cerebral vascular disease in patients of these four types.